Much of the prior art of matrix fixturing centers on the use of low melting metallic alloys having controlled shrinkage characteristics. In this connection reference may be had to U.S. Pat. Nos. 3,790,152 and 3,982,430. These materials provide a matrix for holding a wide range of metal parts for drilling, grinding, lapping, milling and turning operations. Certain metallic alloys in which bismuth or antimony are incorporated give molten metal alloys which expand upon solidification thus providing excellent duplication and reproduction. There are, however, many commercial operations where these metallic alloys do not provide optimum results. The weight of these alloys when used in conjunction with large parts poses a processing problem; their high cost makes for an expensive inventory problem, and the loss of any of the alloy during processing is a serious economic disadvantage. Contamination caused by metal such as bismuth, cadmium, lead and tin contained in such materials is extremely damaging in the encapsulating of jet engine parts, for example.
One of the major obstacles to wider utilization of thin wall, odd-shaped, fragile and nonmagnetic parts has been the relatively inflexible method by which they are mechanically encapsulated for machining. Production difficulties in supporting these parts are caused by vibration and chatter, distortion, fracturing, deflection, interrupted cuts, rapid consumption of cutting tools, heavy burring, high scrap rate and secondary repairs. Typical material requirements for jet engine blade staging are as follows:
(1) The material should have the rigidity of lead at room temperature or below. PA1 (2) The material should be injectable and have a set up time of about 2-3 minutes. PA1 (3) The material should have minimum shrinkage and not be appreciably soluble in machining coolants. PA1 (4) The material should be easily removable from machined parts in such a manner as not to damage the parts. Any residual material should not attack the parts or workpieces or coatings thereon at temperatures as high as 2000.degree. F. PA1 (5) the material should be nontoxic, nonflammable and relatively odorless. PA1 (6) The material should be relatively inexpensive and reusable. PA1 (7) The material should have good shelf life and a pot life at application temperature.
Encapsulating a workpiece is different from "potting" a workpiece. In the former, the entire workpiece is enclosed by a resinous composition. In the present invention, this is done by injection molding the resinous composition at an elevated temperature into a closed mold cavity in which the workpiece is precisely positioned. After cooling the mold, the workpiece and its entirely surrounding hardened resinous composition is withdrawn from the mold and submitted to a machining operation. As the workpiece is machined, part of the encapsulating compound is machined away from the workpiece. In the case of potting, the workpiece is first accurately located in a matrix box with both ends of the workpiece projecting out of the box. The compound is then poured into the box and solidified with one or both ends of the workpiece remaining exposed. One or both ends of the workpiece may then be submitted to a metal forming or shaping operation without the tool engaging the potting compound.
Because of the difference between potting and encapsulating, different compositions are utilized. The encapsulating composition must be stronger in order to withstand the load forces applied directly to the encapsulating compound.
Heretofore, organic compositions have been used as workholding compositions. For example, reference may be had to the patent to Lapac et al, U.S. Pat. No. 3,897,535. This patent discloses a process for fixturing a workpiece including disposing a liquid organic resin work holding material within the work holder around the workpiece and quenching the assembly at between 40.degree. F. and 73.degree. F. to solidify the work holding material. The cold quench within this particular temperature range gives improved holding strength and reduces shrinkage of the organic work holding material. According to the Lapac process, ceramic stones are heated and disposed within the work holder to heat the work holder and the workpiece prior to pouring the work holding composition into the work holder. The stones further increase the holding strength of the work holding material and reduce shrinkage even more. Glass balls may be used in place of ceramic stones. These materials are, however, of substantial diameter, preferably between one-eighth and two-tenths of an inch in diameter. Smaller sized balls do not permit proper flow of wax into the work holding cavity. Stones or balls of a greater size while they may perform satisfactorily in heating the workpiece do not add much strength to the solidified composition. The organic resin material according to Lapac may be any wax or thermoplastic which is solid at room temperature.
Other references of interest in this field include Speyer U.S. Pat. Nos. 3,921,343; 3,748,155 and 3,854,962; Shepard 3,586,559 and McCormick 3,319,289.
The present invention is an improvement in the use of organic materials for fixturing workpieces, particularly encapsulating workpieces. Because of the composition and the process of the present invention utilized herein, the operations of separately filling a work holder cavity with the ceramic stones or glass balls followed by pouring or injecting a molten composition thereover is avoided. Moreover, the necessity for recovering the glass balls or ceramic stones is obviated thereby reducing the number of operations and the attendant cost of recovery of materials. The problems of handling of very hot (800.degree. F.) stones are avoided. The compositions of the present invention have all of the desired properties for encapsulating of devices such as jet engine blades while achieving these objectives in a more economical and useful manner. The filler material does not need to be removed from the composition in order that the composition may be properly reused. The compositions hereof have excellent shelf life, and even when molten undergo minimum settling of the inorganic moiety.